Mr. HatchettV Ohfervatkns on Bituminous Subjiances. 135 



hums well, and is fo compact that it is often employed, like jet, to 

 be formed into trinkets. 



The great refemblance which cannel coal has to jet in many of 

 its properties, induces me to regard it as the next gradation of the 

 compound bituminous fubftances, and as the leading variety of coal 

 from which the others follow according to the decree of their 

 bituminous character. 



The limits of this paper will not allow me to enter into a cir- 

 cumftantial account of all the other varieties of pit-coal ; neither 

 is it neceiTary, after the gradations of afphaltum to jet, and of jet 

 to coal, have been noticed. I fhall not therefore defcribe the varie- 

 ties of coal known by divers names in different countries, and even 

 in different provinces, fuch as thofe called in England caking coal, 

 rock coal, fplent coal, &c. &c. ; but fhall only obferve, that the 

 pit-coals in general appear to be compofed of bitumen intimately 

 mixed, or rather combined, with various proportions of carbon and 

 earthy matter ; and according to the intimacy of the union, and the 

 excefs of one or other of the ingredients, fo the compound poffefles 

 more or lefs the characters of perfect coal, or, by various fhades, 

 paffes into certain earthy or ftony fubftances, which, although im- 

 pregnated with bitumen, do not merit the appellation of coal, and 

 thefe alfo at length gradually lofe the bituminous character*. 



, It is likewife worthy of notice, that the quantity of earthy mat- 

 ter does not appear to be the principal caufe why pit-coals do not 

 burn with the rapidity which is to be perceived in fome other earthy 

 fubftances impregnated with bitumen. For we may conclude, that 



* From Mr. Kirwan's experiments it appears that carbon is a conftituent principle 

 of coal, and that the prefence of it is a principal caufe of thofe modifications which pro- 

 duce the fpecies. It even feems chiefly to form the Kilkenny coal. — Kirwan's Elements 

 ,f Mineralogy, vol. ii. p. 521. 



the 



